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  Java Practices -> Prefer empty items to null ones
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<div class='page-title'>Prefer empty items to null ones</div>

<div class='main-body'>
 
<br>There are many cases in which an "empty" object can be used instead of a null object reference.
This is usually preferable, since it helps eliminate one of the most common problems encountered during 
development -- the <a href='http://java.sun.com/javase/6/docs/api/java/lang/NullPointerException.html'>NullPointerException</a>.

<P>The following qualify as "empty" objects :
<ul>
 <li>the empty String
 <li>zero-length arrays
 <li>collections containing 0 items
 <li>maps containing 0 items
</ul>

<P>Example declarations of empty items :
<PRE>
<tt>String item = "";</tt>
<tt>BigDecimal[] items = new BigDecimal[0];</tt>
<tt>List&lt;Integer&gt; items = new ArrayList&lt;Integer&gt;();</tt>
<tt>Map&lt;String, Integer&gt; items = new LinkedHashMap&lt;String, Integer&gt;();</tt>
</PRE>


<P>The <tt><a href="http://java.sun.com/javase/6/docs/api/java/util/Collections.html">Collections</a></tt>
class contains several type safe methods which return empty items (which are also immutable and <tt>Serializable</tt>) :
<ul>
<li><tt>Collections.<a href='http://java.sun.com/javase/6/docs/api/java/util/Collections.html#emptyList()'>emptyList()</a></tt></li>
<li><tt>Collections.<a href='http://java.sun.com/javase/6/docs/api/java/util/Collections.html#emptySet()'>emptySet()</a></tt></li>
<li><tt>Collections.<a href='http://java.sun.com/javase/6/docs/api/java/util/Collections.html#emptyMap()'>emptyMap()</a></tt></li>
</ul>
The above methods are occasionally useful.


<p><b>Example</b>
<P>Here's an example of using an empty, zero-length array instead of a null reference.
This simplifies the caller, since they never have to check for <tt>null</tt> values. 
(Another alternative is to use replace the array with a possibly-empty collection.)

<br>
<PRE>

<span class='keyword'>import</span> java.util.*;

<span class='keyword'>public</span> <span class='keyword'>class</span> StockPortfolio {

  <span class='comment'>//..elided
</span>
  <span class='comment'>/**
  * A field is implemented internally as a Collection,
  * but offered to the user of this class as an Array.
  * This array is never null, even if the underlying Collection is
  * empty.
  */</span>
  <span class='keyword'>public</span> String[] getStocks() {
    <span class='comment'>/*
    NO_STOCKS plays a double role here. If there are elements in fStocks, then
    toArray will allocate a new array, using NO_STOCKS simply as an indicator
    of the proper type to use. If fStocks has no elements at all, then toArray
    will simply return the NO_STOCKS array itself.
    */</span>
    <span class='keyword'>return</span> (String[]) fStocks.toArray(NO_STOCKS);
  }

  <span class='comment'>// PRIVATE //
</span>
  <span class='comment'>/** The underlying collection.  */</span>
  <span class='keyword'>private</span> List fStocks;

  <span class='comment'>/** A constant, empty array, to be used instead of a null array.  */</span>
  <span class='keyword'>private</span> <span class='keyword'>static</span> <span class='keyword'>final</span> String[]  NO_STOCKS = <span class='keyword'>new</span> String[<span class='literal'>0</span>];
} 
</PRE>
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<div class='topic-section'>See Also :</div>
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  <a href='TopicAction55c9-2.html?Id=39'>Prefer Collections over older classes</a> <br>
 
  
  <a href='TopicAction772f-2.html?Id=134'>Avoid null if possible</a> <br>
 
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